Windowpanes for building materials etc. are required to have a heat insulating property from the viewpoints of energy saving, CO2 reduction, and dew condensation prevention, etc. Multilayer glass panels, in each of which a heat ray reflective film is formed on an inner surface of the panel, have been widely developed as the heat-insulating members of the windowpanes, and the spread of the panels has been greatly increased. Such a heat-insulating member, i.e., a multilayer glass panel has a structure in which: a space is provided between a pair of glass substrates; the space has an atmosphere filled with dry air having heat conductivity lower than that of the atmospheric air containing moisture or with argon, or in a vacuum state; and the outer peripheral part of the panel is sealed with a resin or low-melting glass. The heat conductivities of these atmospheres satisfy the relationship of dry air>argon>>vacuum, and the heat insulating property of the panel is increased in the order of dry air<argon<<vacuum. The heat conductivity at 0° C. is 0.0241 W/mK with dry air, 0.0164 W/mK with argon, and 0.0005 W/mK with vacuum. A metal thin film is generally used as the heat ray reflective film, which is often formed on the inner surface of one of the glass substrates. A heat ray reflectance can be improved by forming the metal thin films into a multilayer via oxide thin films. Soda-lime glass, having heat resistance of approximately 500° C., is generally used as the glass substrate, and hence it is necessary to seal the outer peripheral part of the glass substrate at least under a condition in which the temperature is not exceeded.
When the inside of the panel is in a vacuum state, glass sealing using a low-melting glass is adopted to achieve a heat insulating property higher than that of a sealing agent using a resin. A PbO—B2O3-based low-melting glass or Bi2O3—B2O3-based low-melting glass is used in the glass sealing. However, the PbO—B2O3-based low-melting glass contains a large amount of harmful lead (Pb), and hence such a low-melting glass cannot be used in electronic and electrical appliances because of environmental regulations, such as RoHS Directive. Building materials have also been required to be lead-free in order to reduce influence on environmental impact. Additionally, it comes to be expected that vacuum heat insulation multilayer glass panels will be developed into refrigerators, freezers, electronic and electrical appliances, and the like, other than windowpanes for building materials, and for such development, it is absolutely necessary to make a glass sealing part lead-free.
On the other hand, the Bi2O3—B2O3-based low-melting glass can be made lead-free, and it has already been wholly adopted in electronic and electrical appliances, instead of the PbO—B2O3-based low-melting glass. However, the Bi2O3—B2O3-based low-melting glass is softened and fluidized at a temperature higher than that of the PbO—B2O3-based low-melting glass, and hence a multilayer glass panel is airtightly sealed at a temperature near to the heat-resistant temperature of soda-lime glass used as a glass substrate. This makes the production yield of a multilayer glass panel decrease, and hence the Bi2O3—B2O3-based low-melting glass is required to have a softening temperature as low as that of the PbO—B2O3-based low-melting glass. Additionally, the Bi2O3—B2O3-based low-melting glass contains a large amount of bismuth (Bi), the amount of deposit of which is small, and hence there is the possibility that the continuous and stable supply of Bi may become difficult. Further, Bi is often produced along with by-products of lead, and a large amount of harmful lead wastes are discharged when purified, and hence Bi is not necessarily environmentally-friendly.
Patent Literature 1 proposes that the outer peripheral part of a vacuum heat insulation multilayer glass panel is airtightly sealed by using a lead-free V2O5—BaO—ZnO-based low-melting glass. It is disclosed that the composition of the V2O5—BaO—ZnO-based low-melting glass suitable for the airtight sealing contains 45 to 50 mol % of V2O5, 20 to 23 mol % of BaO, and 19 to 22 mol % of ZnO.